The present invention relates to a process for purifying sulfuric acid containing boron, and more particularly, the present invention relates to a process for purification of by-product sulfuric acid from boron trifluoride synthesis.
It is known that anhydrous boron trifluoride can be obtained by the reaction of substantially anhydrous hydrofluoric acid on orthoboric, tetraboric or metaboric acids or on boric anhydride through the use of sulfuric acid in the presence of sulfuric anhydride introduced as such or in the form of oleum so as to maintain the reaction medium in a substantially anhydrous condition to produce anhydrous boron trifluoride. The use of sulfuric anhydride for dehydrating the reaction medium results in the formation of by-product sulfuric acid which must be purged. This acid, slightly contaminated with fluorine and boron compounds, cannot directly be used.
Various methods have been proposed to purify the sulfuric acid by-product from the synthesis of boron trifluoride. In German OS No. 2,027,389 a treatment with an inert gas leads to a sulfuric acid containing about 0.03 to 0.07% boron. Further, in British Pat. No. 2,050,328, the treated sulfuric acid still contains about 0.015 to 0.1% boron, even though the conditions (namely, an atomic ratio of fluorine to boron of 3 or greater) are such that the elimination of boron ought to be more favorable.
None of these prior art methods proving satisfactory, it has presently been found that a process for the purification of sulfuric acid by-product from boron trifluoride synthesis permits obtaining a quality almost equivalent to that of sulfuric acid made by the oxidation of sulfur, that is to say, a sulfuric acid containing less than 0.003% of fluorine and less than 0.005% of boron. Although it is known that to eliminate the boron contained in sulfuric acid it is necessary that it be in the form of boron trifluoride, that is to say, that the atomic ratio of fluorine to boron is greater than 3 (because when this condition is not met, the boron remains in part in the state of difluodihydroxoboric acid which cannot be eliminated from the medium by entrainment), this condition, as brought out notably in British Pat. No. 2,050,328, is insufficient to obtain sulfuric acid having the desired purity.